The present invention relates to fluid dispensing apparatuses, and more particularly to a volumetric fluid dispensing apparatus having no moving parts in contact with the fluid and which can deliver a wide range of volume of fluids.
Numerous types of fluid dispensing apparatuses exist for filling bottles. One type of fluid dispensing apparatus which is in widespread use is positive displacement fillers. Positive displacement fillers typically include moving parts which contact and displace the fluid being dispensed. For example, one type of positive displacement filler uses a piston and cylinder arrangement. In this type of positive displacement filler, the backward movement of the piston draws fluid into the cylinder through an inlet port and the forward movement of the piston expels the fluid through an outlet port. Another type of positive displacement filler uses a rotary pump to move the fluid.
Positive displacement pumps have gained widespread use in the United States for two reasons. First, positive displacement pump can operate at relatively high speeds, filling as many as six hundred bottles per minute. Additionally, positive displacement pumps are accurate up to about ±0.5%.
Despite the widespread use of positive displacement fillers, they nevertheless have several disadvantages. One disadvantage with positive displacement fillers is that the fluid comes into contact with moving parts. As the moving parts wear, particulate matter enters the fluid causing particulate contamination. If severe enough, particulate contamination can render the product unusable. Another disadvantage with positive displacement fillers involves the difficulty in cleaning and sterilizing the moving parts in contact with the fluid. In positive displacement pumps, the critical tolerances between pads, such as the piston and cylinder, preclude effective cleaning in place. Thus, the user must disassemble the apparatus for cleaning and sterilization. This process is not only time consuming, but may result in biological contamination of the pads when they are handled by the mechanic during reassembly.
It has been proposed in U.S. Pat. Nos. 5,480,063 and 5,680,960 to provide volumetric fluid dispensing apparatus which utilizes gravity rather than apparatus having moving parts in contact with the fluid being dispensed such a pump. A closed fluid chamber is provided in which the fluid level therein is sensed and maintained at a constant level. A fill tube in fluid communication with the fluid chamber for containing a desired volume of fluid is provided. Sensing apparatus which control valve apparatus is provided for filling the fill tube with a desired volume of fluid from the fluid chamber and subsequently dispensing the desired volume of fluid into a container such as a vial. In these apparatus, the top surface of the fluid being dispensed from the fill tube must be lower than the top surface of the fluid in the fluid chamber in order to effect dispensing of the desired fluid volume. Since the fill tubes disclosed in these patents have a constant inner diameter along their length, it has been found necessary to utilize a plurality of fill tubes of varying inner diameters in order to effect dispensing of a desired range of fluid volumes, e.g., 0.5 ml–20 ml. The use of a plurality of fill tubes in a single dispensing apparatus or a plurality of dispensing apparatus, each having a fill tube with a given constant diameter since such an arrangement is either needlessly complicated or needlessly expensive.
In the current system, the dispense volume repeatability is critical. This repeatability is expressed as a percent error of the fill volume. The error in the dispense volume is affected by the sight tube diameter at the sensor locations. The larger the inner diameter, the more volume/height of the tube. Since the sensors detect the level of the liquid and the inherent error in detecting the liquid level at the same location for consecutive dispenses results in different dispense volumes, it is preferred to have the inner diameter of the sight tube be small so that the volume/height ratio is small. This approach will result in the most repeatable fill volumes. As an example, having a sight tube with a 0.5 inch inner diameter the volume for every 0.001 inch of height is 0.0032 cc whereas a sight tube of 0.25 inch inner diameter will have a volume of 0.0008 cc for every 0.001 inch of height. For this reason one would want to have a small inner diameter sight tube for low volume dispense volumes. Likewise it will be beneficial to have larger inner diameter sight tubes for larger volumes for capacity and speed while the error may be larger in absolute volume but it will be small as a percentage of the fill. The tapered sight tube of this invention allows the filling of a wide range of volumes while maintaining the required repeatability since the sensors will be located at the preferred inner diameter for the required dispense volume.
Accordingly, it would be desirable to provide a fluid dispensing apparatus capable of accurately dispensing fluid over a wide volume range without the need for a plurality of fill tubes (with various inner diameters). Such an apparatus would simplify dispensing of fluids.